1. Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus, and more particularly, to a mechanism for performing a position correction in a main-scanning direction and a skew correction at a leading edge of a sheet.
2. Description of the Related Art
An image forming apparatus such as a laser printer feeds sheets such as printing papers accommodated in a feeding unit one by one, transfers a toner image formed on a photosensitive drum, a photosensitive belt, or the like onto the sheet at a transfer position, and fixes the toner image to the sheet, thereby obtaining the sheet with the toner image thereon.
In such image forming apparatus, a registration mechanism including a stopper and rollers is arranged just before the transfer position to correct the direction of the sheet, so that the toner image can be transferred at an appropriate position on the sheet.
For example, in Japanese Patent No. 2893540, the image forming apparatus includes a sheet conveying device. In the sheet conveying device, a stopper for positioning a sheet in a direction orthogonal to a sheet conveying direction is provided on a conveying path, and a leading end of the sheet is brought into contact with the stopper, so that the sheet is stopped. In this state, the sheet is fed by a conveying unit on the upstream side, and the stopper is released after a loop is formed in the sheet, so that the sheet is nipped and conveyed by a pair of rollers downstream of the stopper. Thereafter, a detecting unit that is arranged downstream of the stopper detects a side edge of the sheet, and a roller moving unit moves the rollers in a direction orthogonal to the sheet conveying direction to correct the position of the sheet so that the side edge of the sheet is aligned with a reference position.
FIG. 9 is a schematic diagram of a conventional sheet conveying device that includes a pair of lateral registration rollers 32, a stopper 33, a pair of feeding rollers 34, a sheet edge detection sensor 35, pairs of conveying rollers 36, sheet conveying paths 37 and 38, and sheet trays 40 and 41.
The stopper 33 is arranged just upstream of the lateral registration rollers 32, and can move between a sheet-conveying-path closed position and a sheet-conveying-path opened position. The distance between the lateral registration rollers 32 and the feeding rollers 34 in the sheet conveying path is shorter than a small-size sheet for enabling them to convey the small-size sheet, and the sheet conveying path upstream of the feeding rollers 34 includes the sheet conveying path 38 connected to the sheet tray 40 arranged in the apparatus body and the sheet conveying path 37 connected to the sheet tray 41 arranged outside the apparatus body. The pairs of conveying rollers 36 are arranged along the sheet conveying paths 37 and 38 for conveying the sheet to the feeding rollers 34. Moreover, the sheet conveying paths 37 and 38 are joined at a sheet-conveying-path junction point D upstream of the feeding rollers 34.
The operations of a sheet conveying position correction and a sheet skew correction in the above sheet conveying device are explained.
A sheet 39 conveyed by the feeding rollers 34 is stopped after the leading end thereof comes into contact with the stopper 33 that is set to the sheet-conveying-path closed position in advance. At this time, because the leading end of the sheet 39 is aligned with the stopper 33, the sheet skew correction is finished. Thereafter, the sheet 39 is fed by the feeding rollers 34 for a while until a buffer C is formed in the sheet 39 between the stopper 33 and the feeding rollers 34. Then, the stopper 33 is lowered to release the leading end of the sheet 39. In this state, due to the stiffness of the sheet 39 at the buffer C, the leading end of the sheet 39 is pushed into the nip portion of the lateral registration rollers 32. At this time, the nipping by the feeding rollers 34 is released, and the edge (side edge) of the sheet 39 in a main scanning direction is detected by the sheet edge detection sensor 35. After the correction amount of the sheet 39 in the main scanning direction is calculated, the lateral registration rollers 32 are moved laterally in an axis direction of the lateral registration rollers 32 by the correction amount, thereby aligning the position of the sheet 39 in the main scanning direction without the feeding rollers 34 affecting the operation of the sheet position correction (lateral registration).
When the sheet conveying position correction and the sheet skew correction are performed for a sheet having a length longer than the distance between the lateral registration rollers 32 and the conveying rollers 36 in the above sheet conveying device, if the conveying rollers 36 nip (press and hold) the trailing end of the sheet even after the leading end of the sheet is pushed into the nip portion of the lateral registration rollers 32, the sheet may be skewed to wrinkle or the sheet whose skew has been corrected by the stopper 33 may be skewed again due to the resistance at the nip portion between the conveying rollers 36 at the time of laterally moving the sheet with the lateral registration rollers 32 for the sheet conveying position correction. Therefore, when the sheet conveying position correction is performed, the conveying rollers 36 are released.
At the time when the stopper 33 is lowered to release the leading end of the sheet after forming the buffer C in the sheet between the stopper 33 and the feeding rollers 34, if the sheet is curled or has a low stiffness, the sheet may be buckled or skewed before being nipped by the lateral registration rollers 32, thereby misaligning the direction of the sheet or causing a jam of the sheet. On the contrary, if the sheet has a high stiffness, the sheet whose skew has been corrected by the stopper 33 may be skewed again and pushed into the nip portion of the lateral registration rollers 32 in this state, which indicates that the skew correction by the stopper 33 has no meaning. To solve this problem, the stopper 33 is arranged downstream of the lateral registration rollers 32 (see, for example, Japanese Patent Application Laid-open No. H10-203690).
In addition, with the above configuration, because the stopper and the conveying unit each need a driving unit, the apparatus itself becomes large and the manufacturing cost thereof becomes high.
Even if the above problems are solved, in the case of conveying the sheet that is thick, stiff, and has a length longer than the distance between the lateral registration rollers 32 and the sheet-conveying-path junction point D, if the radius of curvature of the sheet conveying path 38 from each sheet tray to the feeding rollers 34 is too small, the rear end portion of the sheet remaining on the sheet conveying path receives high resistance in conveying on the sheet conveying path. Consequently, when the sheet conveying position correction of the sheet 39 is performed in the main scanning direction by the lateral registration rollers 32, the resistance on the sheet conveying path interferes with the movement of the sheet 39 in the main scanning direction, which results in lowering the accuracy of aligning a sheet in conveying.
In the conventional sheet conveying device in FIG. 9, for performing the sheet skew correction by making the leading end of the sheet in contact with the stopper 33 and the sheet conveying position correction in the main scanning direction by the lateral registration rollers 32, it is necessary to associate the open/close timing of the conveying path by the stopper 33 and the nipping timing of the sheet by the lateral registration rollers 32 and the conveying rollers 36 with each other. That is, as described above, if the conveying rollers 36 nip the trailing end of the sheet after the leading end of the sheet is pushed into the nip portion of the lateral registration rollers 32, the sheet may be skewed to wrinkle or the sheet whose skew has been corrected by the stopper 33 may be again due to the resistance at the nip portion between the conveying rollers 36 at the time of performing the sheet position correction in main scanning direction by laterally moving the lateral registration rollers 32. Therefore, there is considered a configuration for operating each member at constant timing, in which a plurality of cams is provided on the same cam shaft to prevent a size increase of the apparatus, and the open/close timing and the nipping timing by the members are defined by rotating the cam shaft at a constant speed.
However, when the cams defining the timing of the members are used, the opening/closing operation and the nipping operation are performed by the members at constant timing based on the constant rotation speed of the cam shaft. Therefore, if there are various types of sheets with different lengths, unnecessary time loss is generated in conveying depending upon the length of the sheet, which may lower the conveying efficiency.
Specifically, in the above sheet aligning unit, in the case of performing the sheet skew correction and the lateral registration of the sheet while conveying the sheet at constant timing, the sheet aligning unit is control to operate normally for a sheet having the maximum length in the sheet conveying direction available for the sheet aligning unit. For example, if the sheet aligning unit is used in the image forming apparatus in which an A4 size sheet is available at the maximum length, because there are not so many types of sheets having a length shorter than the A4 size sheet used, the sheet aligning unit can be easily controlled by causing it to operate at constant timing corresponding to the length of the A4 size sheet.
However, in practice, there are many types of sheets having different lengths available. If the difference in length between longer and shorter sheets is large, the shorter sheet is controlled to be conveyed at the same timing as that for the longer sheet. Therefore, for example, the feeding rollers that nip and convey the trailing end of the sheet come into contact with each other again at the timing for the longer sheet even when conveying the shorter sheet. Therefore, the waiting time in the case of the shorter sheet becomes long compared to the case of the longer sheet, thereby causing a time loss.